Describe the differences in the arrangement and movement of particles in a solid compared to a gas, and use the particle model to explain why a gas can be compressed but a solid cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bicycle pump contains air that can be squeezed into a smaller volume when the handle is pushed in. However, the metal cylinder of the pump itself cannot be squeezed in the same way.
Model answer (4 marks)
In a solid the particles are arranged in a regular, tightly packed lattice and vibrate about fixed positions, so they have very little free space between them. In a gas the particles are spread out, randomly arranged and move rapidly in all directions, leaving large gaps between them.
Because a gas has these large spaces, the particles can be pushed closer together when pressure is applied, so the gas can be compressed. In a solid the particles are already as close as they can be and there are no significant gaps, so applying pressure does not reduce the volume appreciably.
Because a gas has these large spaces, the particles can be pushed closer together when pressure is applied, so the gas can be compressed. In a solid the particles are already as close as they can be and there are no significant gaps, so applying pressure does not reduce the volume appreciably.
Examiner tips
- Use the terms ‘regular arrangement’ and ‘tightly packed’ for solids, ‘randomly arranged’ and ‘large gaps’ for gases. Mention particle vibration and energy levels. Explain compression by referring to gaps. Keep answer to 4 marks.
Common mistakes
- Confusing the arrangement of particles in solids and gases. Failing to mention that solids cannot be compressed because particles are already close together. Using vague phrases like ‘particles move’ without specifying energy or arrangement.
Mark scheme (4 marks)
- In a solid, particles are held in a regular arrangement; in a gas, particles are relatively spread out / randomly arranged.
- Particles in a solid have the least energy and vibrate about fixed positions; particles in a gas move randomly in all directions with the most energy.
- A gas can be compressed because there are large spaces / gaps between the particles, so the particles can be pushed closer together.
- A solid cannot be compressed because the particles are already close together / there are no significant gaps between particles.
Key terms in this question
particle model · solid · gas · compressed · arrangement
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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